pubs.acs.org/Langmuir © 2010 American Chemical Society
Pickering Emulsion as a Template to Synthesize Janus Colloids with Anisotropy in the Surface Potential Neeraj P. Pardhy and Bridgette M. Budhlall* Center for High-Rate Nanomanufacturing and Department of Plastics Engineering, University of Massachusetts, Lowell, Massachusetts 01854 Received April 16, 2010. Revised Manuscript Received June 14, 2010 A versatile new concept is presented for the synthesis of Janus colloids composed of Laponite nanoclay armored poly(divinylbenzene) with an anisotropic surface potential via a double Pickering emulsion template. First, polystyrene or poly(divinylbenzene) colloids stabilized with Laponite nanoclay are synthesized via a Pickering miniemulsion approach. These nanoparticle-stabilized colloids were then templated at a wax-water interface in a second Pickering emulsion in order to chemically modify one hemisphere of the colloids. Janus modification of the colloids was accomplished by cation exchange of sodium ions, originally present on the surface of the Laponite with various salts of modifying ions (Ca2þ, Fe2þ, and Fe3þ) in the suspension. The zeta potential of the chemically modified and unmodified colloids was compared. The maximum change in the zeta potential was given by the calcium ions, Ca2þ-adsorbed modified colloids as compared to unmodified sodium ions, Naþ-adsorbed colloids. The distribution of charges on the Janus colloids results in a nonuniform zeta potential. XPS and optical microscopy were used to verify the successful chemical modification by the cation exchange of Naþ for Ca2þ ions on one hemisphere of the Janus colloids.
Introduction Particles having anisotropic properties are called Janus particles.1 Particles may be anisotropic in shape and/or surface chemistry. Particles with chemical rather than shape anisotropy have potential importance and can play a role in recognizing specific molecules,2-4 self-assembling colloids,5,6 and stabilizing bubbles.7 It is important to develop methods for fine tuning both the geometry and chemical compositions of anisotropic particles. Moreover, to make these particles practical, they should be produced in bulk rather than on a particle-by-particle basis.8 High volumes of anisotropic colloids with precisely controlled surface potential and functionality cannot currently be produced. The synthesis of Janus colloids with differential surface potentials in large quantities with a bulk processing method has not yet been successfully accomplished. Although methods such as one-sided gold evaporation,9 colloidal crystallization,10 microcontact printing,11 and electrified jetting12 have been studied for the fabrication of various Janus particles, difficulties in scaling up the fabrication preclude the widespread use of these techniques. The first difficulty with these approaches is that an adhesion layer must be applied between the particle and the substrate,11 which greatly limits the size and the geometry of the Janus particles that can be synthesized. A second limitation is that a cleaning step is needed, *Corresponding author. E-mail:
[email protected]. (1) de Gennes, P. G. Rev. Mod. Phys. 1992, 64, 645–648. (2) Glotzer, S. C.; Horsch, M. A.; Iacovella, C. R.; Zhang, Z. L.; Chan, E. R.; Zhang, X. Curr. Opin. Colloid Interface Sci. 2005, 10, 287–295. (3) Zhang, Z. L.; Horsch, M. A.; Lamm, M. H.; Glotzer, S. C. Nano Lett. 2003, 3, 1341–1346. (4) Glotzer, S. C. Science 2004, 306, 419–420. (5) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708–4710. (6) Murphy, C. J. Science 2002, 298, 2139–2141. (7) Du, Z.; Bilbao-Montoya, M. P.; Binks, B. P.; Dickinson, E.; Ettelaie, R.; Murray, B. S. Langmuir 2003, 19, 3106–3108. (8) Walther, A.; Muller, A. H. E. Soft Matter 2008, 4, 663–668. (9) Takei, H.; Shimizu, N. Langmuir 1997, 13, 1865. (10) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240. (11) Cayre, O.; Paunov, V. N.; Velev, O. D. J. Mater. Chem 2003, 13, 2445–2450. (12) Roh, K.-H.; Martin, D. C.; Lahann, J. Nat. Mater. 2005, 4, 759–763.
13130 DOI: 10.1021/la101502e
after printing, to remove the adhesion layer. Furthermore, a reliance on printing surfactant molecules limits the possible chemical makeup.11 Recently, other methods of fabricating Janus particles in large quantities have been developed more successfully. These, however, are not colloids with anisotropic surface potential but rather the simplest possible form of the anisotropic chemical makeup, the Janus particle, whose surface is divided into two different chemical compositions.13 A great need exists to develop anisotropic colloids via a bulk approach that is a scalable, highly parallel synthesis strategy and permits the formation of colloids with fine control over the surface potential and surface chemistry. A number of methods of synthesizing Janus particles have been explored.14 These methods of producing Janus particles broadly employ shielding a part of a homogeneous particle and then modifying the unshielded part with chemical or physical modifying agents or joining two different materials to form a particle that is anisotropic in surface as well as bulk properties. The partial shielding method involves variants in which particles are laid in a tightly bound monolayer on a substrate or are partially embedded in a matrix. Thus, only one side of the particle is exposed to the modifying agents, giving us Janus particles. The tightly bound monolayer is formed by spin coating or drag coating15 homogeneous particles over smooth surfaces; subsequently, Janus particles are obtained by knocking these particles off of their substrate after the monolayer is modified either by sputtering,16 stamp coating,11 or Langmuir-Blodgett-based methods.17 The tight monolayer shields one side of the particles from being exposed to the modifying agents. Partial shielding of the homogeneous particles can also be brought about by partially embedding a monolayer of (13) Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495–9499. (14) Amar, B. P.; Kretchzmar, I. Macromol. Rapid Commun. 2010, 31, 150–168. (15) Prevo, B. G.; Kuncicky, D. M.; Velev, O. D. Colloids Surf., A 2007, 311, 2–10. (16) Takei, H.; Shimizu, N. Langmuir 1997, 13, 1865–1868. (17) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Langmuir 1999, 15, 4630–4635.
Published on Web 07/15/2010
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particles on a substrate18 or on a Pickering emulsion13,19,20 and then chemically modifying these particles in an aqueous phase or vapor phase.21 The procedure for synthesizing Janus particles by the partial shielding of particles using a Pickering emulsion followed by modification in the aqueous phase is the method that we employ. Pickering emulsions are emulsions stabilized by particles instead of surfactants.22,23 About a century ago, Ramsden23 discovered that fine solid particles can be used as stabilizers in emulsions instead of surfactants. The supracolloidal structures obtained using Pickering stabilization are also referred to as colloidosomes.19,24,25 Pickering emulsions have potential applications in controlled drug delivery and cosmetics, arising from the controllable permeability of the particle shell.26,27 The first reported technique was for the preparation of hollow spheres by the assembly of micrometer-sized sulfate latex particles at octanol-in-water emulsion interfaces.19 Weitz and co-workers first coined the term colloidosome and performed systematic work to prepare these capsules with controlled size, permeability, and mechanical strength.28 Recent innovations in Pickering emulsion synthesis include magnetic colloidosomes29 and templated supracolloidal structures.30 In this article, we report the synthesis of novel Janus colloids with an anisotropic surface potential using a previously documented Pickering miniemulsion approach to form colloids with a polymer core and a nanoparticle shell.31-33 These colloids are further templated onto a second Pickering emulsion for chemical modification. After breaking this emulsion, we obtain Janus colloids with anisotropic surface potentials using different salts to modify the particle surface chemically. The proposed strategies will allow us precise control over surface potential and surface chemistry. To our knowledge, the synthesis of large quantities of Janus colloids with an anisotropic surface potential, allowing a response to external stimuli, with a bulk processing method has not yet been accomplished. We use a two-stage approach to synthesize these colloids. The concept of immobilizing silica particles in a wax was previously reported by Granick et al.13,21 and is based on the preparation of a large number of wax colloids coated with native inorganic silica particles and the sequential modification of both sides of the silica particles with different silanes. In this work, the synthesized Janus silica particles had an amino group attached on one side and the other side was an unmodified silica surface. These Janus particles were dipolar in nature because of amine groups on one side that are positively charged in DI water and the native silanol groups of the silica on the other side that are negatively (18) Paunov, V. N.; Cayre, O. J. Adv. Mater. 2004, 16, 9–10. (19) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006–1009. (20) Liu, B.; Wei, W.; Qu, X.; Yang, Z. Angew. Chem. 2008, 47, 3973–3975. (21) Jiang, S.; Schultz, M. J.; Chen, Q.; Moore, J. S.; Granick, S. Langmuir 2008, 24, 10073–10077. (22) Pickering, S. U. J. Chem. Soc. Trans. 1907, 91, 2001–2021. (23) Ramsden, W. Proc. R. Soc. 1903, 72, 156–164. (24) Velev, O. D.; Nagayama, K. Langmuir 1997, 13, 1856–1859. (25) Velev, O. D.; Furusawa, K.; Nagayama, K. Langmuir 1996, 12, 2374–2384. (26) Subramaniam, A. B.; Abkarian, M.; Stone, H. A. Nat. Mater. 2005, 4, 553–556. (27) Nobel, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N. J. Am. Chem. Soc. 2004, 126, 8092. (28) Hsu, M. F.; Nikolaides, M. G.; Dinsmore, A. D.; Bausch, A. R.; Gordon, V. D.; Chen, X.; Hutchinson, J. W.; Weitz, D. A.; Marquez, M. Langmuir 2005, 21, 2963–2970. (29) Melle, S.; Lask, M.; Fuller, G. G. Langmuir 2005, 21, 2158–2162. (30) Strohm, H.; Loebmann, P. J. Mater. Chem. 2004, 14, 2667–2673. (31) Cauvin, S.; Colver, P. J.; Bon, S. A. F. Macromolecules 2005, 38, 7887–7889. (32) Bon, S. A. F.; Colver, P. J. Langmuir 2007, 23, 8316–8322. (33) Ashby, N. P.; Binks., B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640–5646.
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charged in DI water. Because of opposite charges, these particles formed particle chains. On the contrary, in our work the Janus nature of the colloids is attributed to anisotropy in the surface potential rather than the surface charge. The screening of surface charge by different ions on each hemisphere of the Janus colloids leads to anisotropic Debye lengths, which results in anisotropy in surface potentials. The latter anisotropy manifests itself in the colloids’ nonuniform zeta potential. The Janus colloids in this work are observed to cluster rather than chain in water with a certain conductivity. The novelty of our approach lies both in the use of the nanoparticle-stabilized colloids as a particulate stabilizer for the second-stage Pickering emulsion (wax-in-water) and in the use of cation exchange for anisotropic modification of the nanoparticles adsorbed on the surfaces of the colloids.
Results and Discussion A solid-stabilized emulsion approach22,23 was used to synthesize colloids as well as to modify them to form Janus particles. Initially, a Pickering miniemulsion of styrene or divinylbenzene monomer in water was prepared using Laponite nanoparticles as solid stabilizers. The emulsion was polymerized to obtain polystyrene or poly(divinylbenzene) colloids stabilized by Laponite nanoclay particles. These colloids were subsequently used as particles-of-a-kind to stabilize an emulsion of molten paraffin wax in water, referred to as stage II emulsion. The emulsion was then allowed to cool to fix the colloids at the interface of the wax particles and water. The colloids were modified chemically from the aqueous phase using suitable salts as described later. After modification, the salts were removed by filtration from the emulsion and the wax was removed by washing with a solvent to release the Janus colloids. This simple technique is a modification of the toposelective approach recently demonstrated by Granick et al.13,21 Stage I Pickering Emulsion: Synthesis of Colloids Laponite RD is a synthetic trioctahedric hectorite mineral clay composed of a central octahedral magnesium sheet sandwiched between two tetrahedral silica sheets all bound by oxygen atoms. It has a chemical formula of [Si8(Mg5.45Li0.4)O20(OH)4]Na0.7. The clay platelets are circular and 25-35 nm in diameter with a thickness of approximately 1 nm and a density of 2570 kg/m3.34 The clay has an overall negative charge on the surface with positively charged edges. The negative charge on the surface of the clay is stabilized by positively charged sodium (Naþ) ions adsorbed on the surface of the clay as indicated by the chemical structure. These electrostatically bound ions can be substituted by other positively charged ions in cation exchange reactions.35,36 Exchanging the bound ions will change the surface charge potential of the clays depending on the type of ions exchanged (i.e., the valence and the size of the ions being exchanged). This property of clay makes it possible for the Janus colloids to have anisotropic surface potentials on their hemispheres. The clay gets dispersed in water spontaneously because of the like charges on the clay particles, which stabilizes the dispersion. Varying concentrations (0.025-0.2 M) of sodium salt (Naþ) were added to the dispersion to flocculate the clay platelets and enhance the ability of the clay platelets to form a Pickering emulsion. On addition of salt, in this case NaCl(aq), the charges on the platelets get screened. The screening lowers the zeta potential, (34) Laponite Technical Specification Sheet, Southern Clay Products Inc. (35) Missana, T.; Garcia-Gutierrez, M. Phys. Chem. Earth 2007, 32, 559–567. (36) Kaya, A.; Yukselen, Y. Can. Geotech. J. 2005, 42, 1280–1289.
DOI: 10.1021/la101502e
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Figure 1. Schematic of the synthesis of Laponite nanoclay armored polystyrene or poly(divinylbenzene) colloids by a Pickering emulsion approach. Table 1. Compositions of Pickering Emulsions Used to Study the Variation in Colloid Particle Size experiment series no.
sample name
diameter (nm)
monomer mix (g)
NaCl concentration (M)
A
A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 C4 C5
258.30 177.03 274.03 305.00 289.63 149.33 518.26 800.00 274.03 461.40 296.30 518.26 540.00
3.00 1.01 5.09 9.00 3.01 1.18 5.13 8.16 5.09 5.11 5.13 5.13 5.13
0.000 0.000 0.000 0.000 0.100 0.100 0.100 0.100 0.000 0.050 0.025 0.100 0.200
B
C
thereby reducing the electrostatic repulsion and inducing the agglomeration of the Laponite platelets. Agglomerates of Laponite form a “house of cards” configuration that causes the media to form a viscous gel because of the positively charged edges and the negatively charged surfaces. In doing so, the salt helps to improve the partitioning of the platelets to the monomer-water interface, allowing it to act as a stabilizer for the emulsion. Pickering miniemulsions were formed by dispersing the nanoclay in water, adding salt to the dispersion, adding the monomer mixture, and then sonicating to form the emulsion. The monomer mixture consisted of styrene or divinylbenzene monomer, azobisisobutyronitrile (AIBN) as the oil-soluble initiator, and hexadecane as the hydrophobe to prevent the ripening of the emulsion droplets.37,38 A schematic for the synthesis of the Laponite armored colloids or stage I emulsion is summarized in Figure 1. The emulsion was polymerized by heating in water at 60 °C for 48 h to obtain either polystyrene or poly(divinylbenzene) colloids stabilized by Laponite nanoclay platelets, which lay flat on the oil-water interface.33
Effect of NaCl(aq) Salt Concentration on the Colloids Three series of emulsions were prepared to determine the effect of the salt (NaCl) concentration on the particle size of the colloids. The compositions are shown in Table 1. The monomer mixture for the synthesis of colloids contained styrene or divinylbenzene monomers, 8 wt % of hexadecane as the hydrophobe,37,38 and 3 wt % AIBN as the oil-soluble initiator. The first series of experiments (A) was conducted without NaCl(aq) and with varying concentrations of the monomer mixture. The second series (B) was conducted with 0.1 M NaCl(aq) and varying concentrations of the monomer mixture. The third (37) Blythe, P. J.; Morrison, B. R.; Mathauer, K.; Sudol, E. D.; El-Aasser, M. S. Langmuir 2000, 16, 898–904. (38) Hansen, F. K.; Ugelstad, J. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 3069–3082.
13132 DOI: 10.1021/la101502e
series (C) used varying concentrations of NaCl(aq) and a fixed monomer concentration. All three series used 100 g of water and 0.25 g of Laponite platelets. After sonication to form the emulsions, the emulsions were polymerized and then washed by filtration using a nitrocellulose membrane filter with a 200 nm pore size to remove the excess salt in the aqueous phase. Because the Laponite platelets are ∼20 nm, they were removed while the colloids were left behind on the filter. The particle sizes of the emulsions were measured using a Malvern Zetasizer via a dynamic light scattering method. It was observed that all of the samples gave a unimodal particle size distribution. The histogram for sample A1 is shown in Supporting Information Figure S1. From the particle size measurements, the excess number of Laponite platelets can be calculated by the equation32
Cexcess
3π Fpart ¼ C0 2 Foil
!
h Coil doil
ð1Þ
where C0 is the weight of Laponite platelets added to the emulsion, Fpart is the density of the Laponite platelets, Foil is the density of the monomer mix, doil is the diameter of the colloids, h is the height of the Laponite platelets, Coil is the weight of the monomer mix, and Cexcess is the weight of the excess amount of Laponite that remained in the aqueous phase and did not take part in the emulsion. The second term in eq 1 expresses the weight of Laponite platelets that did take part in stabilizing the emulsion. The weight of excess Laponite particles was calculated to be in the range of 45-60% of the total amount of Laponite added to the emulsion. Therefore, it was necessary to separate the excess Laponite from the colloids formed so that it did not interfere with the Janus modification of the colloids. This was accomplished during the salt filtration step mentioned previously.
Effect of Monomer and NaCl(aq) Concentrations on Colloids In Figure 2a, the effect of monomer concentration on the zeta potential at a fixed NaCl(aq) concentration (0.1 M) is seen. The principal reason for determining the zeta potential of the colloidal dispersions was to obtain an indication of the magnitude of the beginning of the diffuse double layer around the colloids. This can then be used to determine the effect of colloidal surface charge on such things as aggregation behavior and stability. The zeta potential increases up to ∼5 g/100 mL of monomer, after which it becomes constant at ∼60 mV. This indicates that the presence of NaCl(aq) contributes to the stability of the Laponite armored colloids at higher monomer concentrations. The variation in the colloid particle size as a function of monomer concentration without and with 0.1 M NaCl(aq) is shown in Figure 2b. Langmuir 2010, 26(16), 13130–13141
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prevented them from adsorbing at the interface to lie flat adjacent to each other, thereby hindering their ability to stabilize large emulsion droplets. In this case, the platelets stabilize smaller droplets having a greater interface curvature where they could adsorb at an angle to each other (
